1. Field of the Invention
This invention relates to a circuit protection device, and more particularly, to an outage device having an overcurrent protection and an overheat protection, and to a surge protection using the outage device.
2. Description of Related Art
As the electronics circuits have become more complicated, and electronics devices have become more precise nowadays, a power supply has to meet a stricter requirement to work with a electronics circuit. Ideally, the current and voltage from a power supply are supposed to be perfect sine or cosine waves, but this perfect power supply doesn't exist in reality. Normally, the voltage output of a power supply is not very steady, and sometimes, there are even power surge accompanied with the output voltage. This accompanied power surge often causes errors on precise electronics devices or even burns down those electronics devices. To resolve this problem, a surge-absorbing device is normally added into circuit designs to absorb power surge for ensuring the regular operations of the loads. In a real world application, a metal oxide varistor (MOV) is often in parallel with the power supply as a surge-absorbing device. The impedance of a MOV is very high when its cross voltage is below a certain predetermined voltage level, that is, there is almost no current flowing through the MOV, that is, the MOV doesn't affect the load at all. As soon as the cross voltage of the MOV, such as a power surge, exceeds the certain predetermined voltage level, the impedance of the MOV toboggans immediately, and most of the supplied current including the power surge is bypassed by the MOV, as a result, the load is protected by the MOV from receiving a power surge, and the load voltage is then remained under a predetermined voltage level.
Referring to FIG. 1A, a MOV 100 is in parallel with the load. In the presence of a power surge, most of the current from power supply including the power surge bypasses the load through the MOV because the impedance of the MOV toboggans immediately. The voltage across the load is then remained under a predetermined voltage level by using a MOV to bypass power surges.
Generally, the temperature on the MOV is increased because the MOV executes the surge-absorbing operation frequently when the supplied voltage is not steady. The MOV will be burned down if the temperature exceeds a certain value. Hence, how to maintain the continuous operation of a circuit under the protection of a MOV has become a pretty important topic. Not only a high temperature can burn down a MOV, a strong current can damage a MOV as well. Furthermore, the circuit is not certainly opened, that is, the burned MOV can still possibly short the circuit and causes a large current. Moreover, since the characteristics of the MOV are changed after it is burned down, a leaking current might happen and affect the circuit near by. Because the MOV is in parallel with the power supply and used as a circuit protection, it is very important to have the MOV pass the test of UL1449 safety regulation to ensure the quality for providing a better protection to the circuits.
However, the previous UL1449 safety regulation was not very rigid, as a result, burning down and explosion still happened on some passed MOV devices. A burned MOV might short the circuit and further generate an arc that can cause more damage to a circuit. Hence, the UL1449 safety regulation has been reviewed and improved into the currently used second edition UL1449 safety regulation.
Within the new UL1449 safety regulation, there are two additional destructive tests. The first test is to feed a huge pulse (1000 A, 240V) into a MOV to blow it and then observe the damage on the MOV. The second test is to feed a gradual increased current (240V, 0.125 A, 0.5 A, 2.5 A, 5 A) to a MOV, and then check the responses of the MOV. The leaking current has to be below 0.5 mV to pass the safety regulation.
Referring to FIG. 1B, a current fuse 120 is in series with a MOV 100. A current flows through a properly selected current fuse 120 before it is fed into the MOV 100, wherein the current fuse 120 is used as a overcurrent protection of the MOV 100. The specification of the current fuse is selected to meet the actual requirement. When the current from the power supply is higher than a predetermined value, i.e., the breaking point of the current fuse, the current fuse is burned down to open the circuit and current can no longer be fed into the MOV 100. Even though the MOV 100 is also burned down by a big current, the circuit is protected from being damaged by a short current created by a broken MOV. Hence, the primary function of the current fuse 120 is to be used as a fast-breaking fuse that breaks itself immediately in the presence of a big current for avoiding a short current.
Referring to FIG. 1C, a temperature fuse 140 is in series with the MOV 100, so a current fed into the MOV 100 has to flow through the temperature fuse first. On the circuit board layout, the temperature fuse and the MOV are separated by a small distance. The temperature fuse 140 is used to detect the temperature on the MOV 100, because the temperature fuse 140 and the MOV 100 are very close to each other. The specification of the temperature fuse 140, which is used as an overheat protection of the MOV 100, is selected properly according to the actual requirement. As soon as the a detected temperature on the MOV 100 exceed the predetermined temperature, the breaking point of the temperature fuse, the temperature fuse is burned down to open the circuit for preventing the temperature over the MOV 100 from increasing. The distance between the temperature fuse 140 and the MOV 100 can be very small, or even zero that means the temperature fuse 140 is on contact with the MOV 100 for detecting the temperature over the MOV directly. Since the increment of temperature is gradual, the primary function of the temperature fuse 140 is to be used as a slow-breaking fuse which breaks itself when the detected temperature is too high to open the circuit for preventing the MOV 100 from being burned down by high temperature.
In a real application, normally a current fuse or a temperature fuse is in series with a MOV as a fast-breaking overcurrent protection or a slow-breaking overheat protection. Conventionally, besides using current fuse as the overcurrent protection, a fine conductive wire on the circuit board can be used as an overcurrent protection as well. When a high current flows through the fine conductive wire, the fine conductive wire is melted and then forms a outage circuit for protecting the MOV from damage. However, the quality of this method is difficult to be controlled which often leads to a result that the characteristics of the circuit cannot be followed precisely. Furthermore, melting the conductive wire causes a permanent damage that makes it difficult to recover and maintain damaged devices. In addition, a conventional method normally has only either a current fuse or temperature fuse in series with the MOV as a protection, which cannot effectively protect the circuit from both overcurrent and overheat. Even though it is possible that having both current fuse and temperature fuse built into a circuit to provide overcurrent and overheat protection, it increases the complication of a circuit, storing and managing cost, and decreases the competition of products.
According to the foregoing, the conventional method includes at least the following disadvantages:
1. the conventional method can only provide either overcurrent or overheat protection to a circuit, wherein the protection is not enough to protect a circuit. PA1 2. using both current fuse and temperature fuse provides a circuit with a double protection, but it increases the complication of a circuit, storing and managing cost, and decreases the competition of products. PA1 3. using a fine conductive wire on the circuit board as a current fuse is difficult to control the quality of protection and characteristics of a circuit, and causes the difficulty on maintenance.